Workshop Agenda Workshop Agenda I. I. Introductions Introductions II. II. Load Rating Basics Load Rating Basics III. III. General Equations General Equations IV. IV. Load Rating Procedure Load Rating Procedure V. V. Incorporating Member Distress Incorporating Member Distress VI. VI. Posting, Posting, SHV’s SHV’s and Permitting and Permitting VII. VII. Load Rating Example #1 Load Rating Example #1 Simple Span Non Simple Span Non - - composite Steel composite Steel
Transcript
Workshop AgendaWorkshop AgendaI.I. IntroductionsIntroductionsII.II. Load Rating BasicsLoad Rating BasicsIII.III. General Equations General Equations IV.IV. Load Rating Procedure Load Rating Procedure V.V. Incorporating Member Distress Incorporating Member Distress VI.VI. Posting, Posting, SHV’sSHV’s and Permitting and Permitting VII.VII. Load Rating Example #1Load Rating Example #1
Erected over a depression or obstruction Erected over a depression or obstruction such as water, a highway or railway, etc.such as water, a highway or railway, etc.
Has a track or passageway for carrying Has a track or passageway for carrying traffic or other moving loadstraffic or other moving loads
Has an opening measured horizontally Has an opening measured horizontally along center of roadway of 10 feet or morealong center of roadway of 10 feet or more
The opening is measured between The opening is measured between undercopingsundercopings of abutments, spring line of of abutments, spring line of arches, or between extreme ends of arches, or between extreme ends of openings for multiple boxesopenings for multiple boxes
What is Load Rating?What is Load Rating?
•• The process of determining the safe The process of determining the safe live load carrying capacity of a new or live load carrying capacity of a new or an existing vehicular bridge is called its an existing vehicular bridge is called its load rating. load rating.
•• Note:Note: load rating should not be load rating should not be confused with bridge condition rating confused with bridge condition rating which is entered into PONTIS.which is entered into PONTIS.
What is Load Rating?What is Load Rating?
•• The final load rating will be the rating of The final load rating will be the rating of the weakest point of the weakest the weakest point of the weakest member within the bridge.member within the bridge.
What is Load Rating?What is Load Rating?•• Live Loads are transient loads that remain in Live Loads are transient loads that remain in
place for a relatively short time. place for a relatively short time. CarsCarsBussesBussesTrucksTrucksWindWindStream flow pressureStream flow pressureEtc Etc
•• Trucks usually used as Live Load for load Trucks usually used as Live Load for load rating of bridgesrating of bridges
What is Load Rating?What is Load Rating?
•• Live load carrying capacity is based on:Live load carrying capacity is based on:
Bridge inspectionsBridge inspections•• Description of any structural modifications Description of any structural modifications •• Identify any condition changesIdentify any condition changes•• Measurements of any lossesMeasurements of any losses
DecayDecayCorrosionCorrosion
•• Field measurements Field measurements
What is Load Rating?What is Load Rating?
•• Live load carrying capacity is based on:Live load carrying capacity is based on:
•• Live load carrying capacity is based on:Live load carrying capacity is based on:
Engineering judgmentEngineering judgment•• May be used to establish if substructure May be used to establish if substructure
controls ratingcontrols rating•• May be used to establish if certain May be used to establish if certain
superstructure components will not control the superstructure components will not control the ratingrating
What is Load Rating?What is Load Rating?
•• Live load carrying capacity is based on:Live load carrying capacity is based on:
Nondestructive Load testsNondestructive Load tests•• Maybe used when bridge cannot be accurately Maybe used when bridge cannot be accurately
modeled by analysismodeled by analysis•• Owner believes a load test will provide a more Owner believes a load test will provide a more
realistic load capacityrealistic load capacity
What is Load Rating?What is Load Rating?
•• Load ratings may be subdivided into Load ratings may be subdivided into specific types depending on live load specific types depending on live load
Design load rating: The live load model is Design load rating: The live load model is the AASHTO design HS loading. The the AASHTO design HS loading. The rating is expressed relative to a HS20 rating is expressed relative to a HS20 truck.truck.
Legal load rating: Sometimes called Legal load rating: Sometimes called posting rating. The live load model is one posting rating. The live load model is one or more of the “legal trucks”.or more of the “legal trucks”.
What is Load Rating?What is Load Rating?
•• Load ratings may be subdivided into Load ratings may be subdivided into specific types depending on live load specific types depending on live load
Annual Permit load rating: The live load Annual Permit load rating: The live load model represents a truck or class of trucks model represents a truck or class of trucks that may operate under an annual that may operate under an annual overweight permit overweight permit
Single Trip Permit load rating: The live Single Trip Permit load rating: The live load model is the specific overweight load model is the specific overweight permit truck permit truck
What is Load Rating?What is Load Rating?•• Load rating is expressed as a rating factor Load rating is expressed as a rating factor
(RF) or weight in tonnage for a particular live (RF) or weight in tonnage for a particular live load.load.
Generally a RF Generally a RF ≥≥ 1.0 indicates that the bridge has 1.0 indicates that the bridge has adequate live load carrying capacityadequate live load carrying capacity
Generally a RF Generally a RF < 1.0 indicates that the bridge has < 1.0 indicates that the bridge has inadequate live load carrying capacityinadequate live load carrying capacity
A RF is always associated with a particular live A RF is always associated with a particular live loadload
Why do Ratings?Why do Ratings?•• Minnesota Administrative Rule 8810.95 Minnesota Administrative Rule 8810.95
Subp.2 Ratings. Subp.2 Ratings.
Bridges must be rated to determine safe load Bridges must be rated to determine safe load carrying capacity and reported on a structure carrying capacity and reported on a structure inventory sheet.inventory sheet.
•• FHWA National Bridge Inspection FHWA National Bridge Inspection Standards (NBIS)Standards (NBIS)
The MCE requires as part of every inspection The MCE requires as part of every inspection cycle, bridge load ratings to be reviewed and cycle, bridge load ratings to be reviewed and updated updated
Why do Ratings?Why do Ratings?
•• Bridge load ratings reported to the Bridge load ratings reported to the NBI weigh heavily in the determination NBI weigh heavily in the determination of the Sufficiency Ratingof the Sufficiency Rating
•• Used in preparing highway needsUsed in preparing highway needs
•• Used in prioritizing projects Used in prioritizing projects
•• Used in distributing bridge funds to Used in distributing bridge funds to local governmentslocal governments
Why do Ratings?Why do Ratings?
•• To have a consistent summary of load To have a consistent summary of load carrying capacities of all state bridgescarrying capacities of all state bridges
•• Bridge load ratings are used to post Bridge load ratings are used to post bridges and for issuing load permitsbridges and for issuing load permits
•• To monitor safety of bridges over timeTo monitor safety of bridges over time
•• To help determine when rehabilitation or To help determine when rehabilitation or replacement is neededreplacement is needed
When to do Ratings?When to do Ratings?
•• According to FHWA National Bridge According to FHWA National Bridge Inspection Standards (NBIS):Inspection Standards (NBIS):
Ratings should be done when there is any Ratings should be done when there is any relevant changes in condition from:relevant changes in condition from:•• Maintenance or improvement workMaintenance or improvement work•• Strength of membersStrength of members•• Dead loadDead load
Ratings should be reviewed and updated Ratings should be reviewed and updated following bridge inspectionsfollowing bridge inspections
Bridge must be Bridge must be reratedrerated when it is determined that when it is determined that a significant change has occurred in the condition a significant change has occurred in the condition of the bridge.of the bridge.
Bridge must be Bridge must be reratedrerated when the allowable legal when the allowable legal load using the bridge is increased.load using the bridge is increased.
Changes in the rating of a bridge must be Changes in the rating of a bridge must be indicated on the structure inventory sheet.indicated on the structure inventory sheet.
When to do Ratings?When to do Ratings?
•• Increase in Dead LoadIncrease in Dead LoadBituminous OverlayBituminous OverlayGravelGravelIncrease weight of railingIncrease weight of railingNew deckNew deckNew beam or girderNew beam or girder
•• Significant changes in AASHTO Significant changes in AASHTO specifications, state policies or federal specifications, state policies or federal directives. directives.
•• A change in law regulating truck weightsA change in law regulating truck weights
When to do Ratings?When to do Ratings?
•• Loss of capacityLoss of capacityCorrosion or damage to steel structural Corrosion or damage to steel structural elements resulting in section losselements resulting in section lossDecay or damage to timber structural Decay or damage to timber structural elements resulting in section losselements resulting in section lossSpallingSpalling or salt related damages in concreteor salt related damages in concreteBridge hit by an oversize loadBridge hit by an oversize load
When to do Ratings?When to do Ratings?
•• Settlement or movement of a pier or Settlement or movement of a pier or abutmentabutment
•• Repairs or remodelingRepairs or remodeling
•• New bridges are rated before opened New bridges are rated before opened to trafficto traffic
What Code to Use?What Code to Use?
•• FHWA National Bridge FHWA National Bridge Inspection Standards Inspection Standards (NBIS)(NBIS)
Adopted the AASHTO Adopted the AASHTO Manual for Condition Manual for Condition Evaluation of Bridges Evaluation of Bridges (MCE) as a NBIS (MCE) as a NBIS regulationregulation
What Code to Use?What Code to Use?•• The AASHTO Manual for Condition The AASHTO Manual for Condition
Evaluation of Bridges (MCE)Evaluation of Bridges (MCE)MCE 2MCE 2ndnd Edition with 2003 InterimsEdition with 2003 Interims2005 Guide Specifications for MCE and 2005 Guide Specifications for MCE and Load and Resistance Factor Rating Load and Resistance Factor Rating (LRFR)(LRFR)New MCE soon to be released, 2008? New MCE soon to be released, 2008? •• Part A Part A -- LRFRLRFR•• Part B Part B -- Allowable Stress Rating (ASR) & Load Allowable Stress Rating (ASR) & Load
Factor Rating (LFR)Factor Rating (LFR)
What Code to Use?What Code to Use?•• Use AASHTO Bridge Design Use AASHTO Bridge Design
Specifications for all matters not Specifications for all matters not covered in the MCEcovered in the MCE
2002 AASHTO Standard Specifications2002 AASHTO Standard Specifications•• ASRASR•• LFRLFR
•• Uses actual (service) loads to produce Uses actual (service) loads to produce maximum stress in membermaximum stress in member
•• Maximum stress in member not to exceed the Maximum stress in member not to exceed the allowable stressallowable stress
•• Allowable stress is the limiting stress of the Allowable stress is the limiting stress of the material with an appropriate factor of safety material with an appropriate factor of safety appliedapplied
•• Per 2003 MCE 2Per 2003 MCE 2ndnd Edition, timber and Edition, timber and masonry bridges should be evaluated using masonry bridges should be evaluated using ASR. ASR.
Maximum stress under service load
Allowable Stress
Yield Stress=≤
Factor of Safety
Load Factor Rating (LFR)Load Factor Rating (LFR)
•• Uses separate load factors (or safety factors) Uses separate load factors (or safety factors) applied to the actual loadsapplied to the actual loads
•• The magnitude of load factors reflect the The magnitude of load factors reflect the uncertainty in the load calculationuncertainty in the load calculation
•• The effect of the factored loads are not to The effect of the factored loads are not to exceed the strength of the member exceed the strength of the member
Load Factor Rating (LFR)Load Factor Rating (LFR)
•• Per 2003 MCE 2Per 2003 MCE 2ndnd Edition, steel, reinforced Edition, steel, reinforced concrete, and concrete, and prestressedprestressed concrete bridges concrete bridges should be evaluated using LFR. should be evaluated using LFR.
(Actual Loads) × SF(Strength Provided) × SF ≥
Note: Safety Factors (SF) are called strength reduction Note: Safety Factors (SF) are called strength reduction factors and load factors. The SF for load are typically factors and load factors. The SF for load are typically 1.3 for dead load and 2.17 for live load. The SF for 1.3 for dead load and 2.17 for live load. The SF for strength are typically .90 for flexural strength and .85 strength are typically .90 for flexural strength and .85 for shear strength for shear strength
Load Factor Rating (LFR)Load Factor Rating (LFR)
•• LFR is the current standard for MnDOTLFR is the current standard for MnDOTNew bridges are designed using LRFD and New bridges are designed using LRFD and are rated using LFRare rated using LFR
Old rating forms not valid for steel and Old rating forms not valid for steel and concrete (ASR)concrete (ASR)
•• Similar to LFR, but uses statistically based Similar to LFR, but uses statistically based load and resistance factors eliminating load and resistance factors eliminating judgment and past experience to determine judgment and past experience to determine factors factors
•• Load and resistance factors have been Load and resistance factors have been calibrated by trial designs to provide a uniform calibrated by trial designs to provide a uniform level of safetylevel of safety
•• LRF method ensures that only 2 out of 10,000 LRF method ensures that only 2 out of 10,000 bridges will have factored loads greater than bridges will have factored loads greater than the factored resistance of the bridge the factored resistance of the bridge
(Actual Loads) × SF(Strength Provided) × SF ≥
Note: LRFR Safety Factors (SF) for load and strength Note: LRFR Safety Factors (SF) for load and strength are statistically based to provide a uniform level of are statistically based to provide a uniform level of safetysafety
Load TestingLoad Testing•• Is an effective means of evaluating the load Is an effective means of evaluating the load
rating of a bridgerating of a bridge
•• Bridge load testing generally consists of:Bridge load testing generally consists of:Load evaluationLoad evaluation
•• Tests on load and load effects from traffic Tests on load and load effects from traffic Diagnostic load testingDiagnostic load testing
•• Test on effects of a known load on bridge Test on effects of a known load on bridge Proof load testing (should not be used)Proof load testing (should not be used)
•• Test to determine maximum live load bridge can supportTest to determine maximum live load bridge can support
Load TestingLoad Testing•• National Cooperative Highway Research National Cooperative Highway Research
Program (NCHRP) Project 12Program (NCHRP) Project 12--28(13) 28(13) developed a Manual for Bridge Rating developed a Manual for Bridge Rating Through Load Testing Through Load Testing
•• Manual covers:Manual covers:General Load Testing ProceduresGeneral Load Testing ProceduresLoad Test Equipment and MeasurementsLoad Test Equipment and MeasurementsDiagnostic Load TestsDiagnostic Load TestsProof Load TestsProof Load Tests
Diagnostic Load TestingDiagnostic Load Testing•• Some general Some general
procedures:procedures:Obtain strain responses Obtain strain responses from known load. from known load. Structural behavior Structural behavior determined from strain determined from strain data data Develop computer model Develop computer model Simulate load test on Simulate load test on computer model computer model Perform load rating on Perform load rating on calibrated modelcalibrated model
Proof Load TestingProof Load Testing•• Using military tanks for Using military tanks for
proof loadproof loadAnalytical methods can Analytical methods can under estimate the under estimate the actual strengthactual strength
Carrying capacity can Carrying capacity can be larger due to be larger due to unintended composite unintended composite action, contribution action, contribution from sidewalks, rail and from sidewalks, rail and etc..etc..
•• Before October 1, 2010Before October 1, 2010New LRFD bridges and replacement New LRFD bridges and replacement bridges use LFR or LRFRbridges use LFR or LRFR
•• After October 1, 2010 After October 1, 2010 New LRFD bridges and replacement New LRFD bridges and replacement bridges use LRFR bridges use LRFR
•• Bridges that warrant a Bridges that warrant a reratingreratingExisting rating is LRFR, Existing rating is LRFR, reratererate using LRFRusing LRFR
FHWA Policy on Rating Methods FHWA Policy on Rating Methods
•• Bridges that warrant a Bridges that warrant a reratingrerating (cont.)(cont.)No existing rating, use LRFR, LFR or ASR (for No existing rating, use LRFR, LFR or ASR (for timber or masonry bridges)timber or masonry bridges)
Existing rating is LFR or ASR, Existing rating is LFR or ASR, reratererate using LRFR, using LRFR, LFR or ASR (for timber or masonry bridges) LFR or ASR (for timber or masonry bridges)
•• Non NHS bridges load rated prior to January 1, 1994, use Non NHS bridges load rated prior to January 1, 1994, use LRFR, LFR or ASRLRFR, LFR or ASR
•• Non NHS bridges load rated after January 1, 1994, use Non NHS bridges load rated after January 1, 1994, use LRFR, or LFR LRFR, or LFR
Posting analysis can use ASR, LFR or LRFR Posting analysis can use ASR, LFR or LRFR (beyond 2010)(beyond 2010)
FHWA Policy on Rating Methods FHWA Policy on Rating Methods
•• CurrentlyCurrentlyDesign new bridges and replacement bridges Design new bridges and replacement bridges using LRFD and rate LFR, but allow ASR for using LRFD and rate LFR, but allow ASR for masonry and timber bridgesmasonry and timber bridges
•• 2008 2008 -- Mn/DOT to complete studies of LRFRMn/DOT to complete studies of LRFR
Mn/DOT Policy on Rating Mn/DOT Policy on Rating Methods Methods
•• 2009 2009 -- Mn/DOT and NHI provide LRFR trainingMn/DOT and NHI provide LRFR training
•• 2010 2010 -- Mn/DOT to meet FHWA policy on LRFRMn/DOT to meet FHWA policy on LRFR
•• 2011 + 2011 + -- RerateRerate all bridges using LRFR all bridges using LRFR
Mn/DOT Policy on Rating Mn/DOT Policy on Rating Methods Methods
Who is Qualified?Who is Qualified?
•• The person charged with overall The person charged with overall responsibility for loadresponsibility for load--rating bridges rating bridges shall have:shall have:
Professional Engineer LicenseProfessional Engineer License55--years bridge design and inspection years bridge design and inspection experienceexperienceKnowledge and skills for proper evaluationKnowledge and skills for proper evaluation
Who is Qualified?Who is Qualified?
•• The PE’s signature of approval on the The PE’s signature of approval on the rating form holds the same level of rating form holds the same level of responsibility as the Engineer’s responsibility as the Engineer’s signature of approval on the bridge signature of approval on the bridge plans and specificationsplans and specifications
Some Ratings Are Not CurrentSome Ratings Are Not CurrentMore bridges are vulnerableMore bridges are vulnerable
•• Of 14,921 Local Bridges Of 14,921 Local Bridges and Culverts:and Culverts:
2,301 have not been rated for 30 2,301 have not been rated for 30 years years
•• Average operating rating of HS Average operating rating of HS 26 or lower26 or lower
Median age = 65 yearsMedian age = 65 years•• Many may need to be reMany may need to be re--ratedrated
Of 765 that are postedOf 765 that are posted•• 190 (25%) Have Not Been 190 (25%) Have Not Been
Rated in 30 Years and Rated in 30 Years and Postings May Need to be Postings May Need to be loweredlowered
Ideal Load Rating HistoryIdeal Load Rating History•• Bridge BuiltBridge Built
Rated at Design levelRated at Design level•• Gradual CorrosionGradual Corrosion
Rating gradually lowered as section Rating gradually lowered as section reducesreduces
•• Severe DeteriorationSevere DeteriorationPosting at appropriate levelPosting at appropriate level
•• ReplacementReplacementCycle starts over againCycle starts over again
Statewide Bridge Rating Statewide Bridge Rating Program NeedsProgram Needs
•• Office of Inspector General requires FHWA Office of Inspector General requires FHWA to better monitor load ratingsto better monitor load ratings
•• Consultant inspection findings often are Consultant inspection findings often are recommending new bridge ratingsrecommending new bridge ratings
•• Likely to have less posted load limits based Likely to have less posted load limits based only on engineering judgmentonly on engineering judgment
Statewide Bridge Rating Statewide Bridge Rating Program NeedsProgram Needs
•• New ratings are required for significant New ratings are required for significant changes in condition or loading since last changes in condition or loading since last inspectioninspection
•• Continued pressures from truckers may Continued pressures from truckers may require statewide permitting, a one stop shop require statewide permitting, a one stop shop for all load permits in the futurefor all load permits in the future
•• AASHTO introduced the new Special Hauling AASHTO introduced the new Special Hauling Vehicle (SHV) posting vehicleVehicle (SHV) posting vehicle
•• Use federal HBRRP funds to pay for Use federal HBRRP funds to pay for bridge ratings on the local levelbridge ratings on the local level
•• Possible Priorities:Possible Priorities:First, bridges with changed conditionsFirst, bridges with changed conditionsSecond, bridges with possible SHV Second, bridges with possible SHV limitationslimitations
•• Possible Priorities:Possible Priorities:Third, new bridge plans (by 2011, all new Third, new bridge plans (by 2011, all new bridges must be rated under LRFR)bridges must be rated under LRFR)Fourth, Fourth, reratererate the remaining bridges, the remaining bridges, county roads followed by township roadscounty roads followed by township roads
•• If County Engineers support it, the Bridge If County Engineers support it, the Bridge Office and State Aid could pursue.Office and State Aid could pursue.
Truck TrendsTruck Trends•• Trucks are getting heavier and increased Trucks are getting heavier and increased
allowable limits have created greater: allowable limits have created greater: Agency costs for inspecting Agency costs for inspecting
Allowed a weight exception to 90,000 GVW Allowed a weight exception to 90,000 GVW for combination vehicles with 6 axlesfor combination vehicles with 6 axlesFor hauling forest productsFor hauling forest productsAnnual permit of $300Annual permit of $300Winter weight increase to 98,000 GVW Winter weight increase to 98,000 GVW
Truck TrendsTruck Trends
•• Truck Size and Weight CommitteeTruck Size and Weight CommitteeReview of truck weight laws was needed:Review of truck weight laws was needed:•• Truck traffic increasingTruck traffic increasing
Recommendations:Recommendations:•• A request for truck weight increasesA request for truck weight increases
•• Truck Size and Weight CommitteeTruck Size and Weight CommitteeRecommendations:Recommendations:•• Allow 4 new vehicle configurations with Allow 4 new vehicle configurations with
A practice that is putting additional wear A practice that is putting additional wear and tear on our infrastructureand tear on our infrastructure
According to a government study, one 40According to a government study, one 40--ton truck does as much damage to the ton truck does as much damage to the road as 9,600 cars road as 9,600 cars
Truck TrendsTruck Trends
•• Increase high stress cyclesIncrease high stress cyclesHigher loads create larger fatigue stresses Higher loads create larger fatigue stresses which can result in fatigue cracking of which can result in fatigue cracking of materialsmaterials
High stress cycles can lead to High stress cycles can lead to requirements of posting, maintenance, requirements of posting, maintenance, and/or expensive repairs and/or expensive repairs
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